(1) Field of the Invention
This invention relates to a radiation detector for measuring a spatial distribution of radiation in the medical, industrial, nuclear and other fields.
(2) Description of the Related Art
A known radiation detector has a common electrode formed on one surface of a semiconductor sensitive to radiation, with a bias voltage applied to the common electrode, and a plurality of split electrodes formed on the other surface of the semiconductor. Incident radiation generates electric charges within the semiconductor, and these charges are fetched through the respective split electrodes as electric signals. In this way, the radiation detector detects a spatial distribution of incident radiation. Such radiation detectors may be manufactured in various ways which may, broadly, be classified into the following three methods.
Firstly, a semiconductor film is formed on a substrate having split electrodes formed thereon beforehand, and then a common electrode is formed on the semiconductor film. Secondly, a semiconductor film is formed on a substrate having a common electrode formed thereon beforehand, and then split electrodes are formed on the semiconductor film. Thirdly, a common electrode is formed on one surface of a semiconductor crystal substrate, and split electrodes are formed on the other surface of the substrate.
The detector manufactured by the first method has a substrate disposed on the split electrode side. The other two types of detectors, generally, are also used by connecting the split electrodes to an electronic circuit board for processing signals. Since each of the above three types has a substrate disposed on the split electrode side, the conventional radiation detector generally detects radiation incident on the common electrode side.
The conventional detector constructed as described above has the following drawbacks.
The conventional radiation detector noted above has no electrodes for sweeping out the charges having been moved by an electric field to regions of space between the split electrodes. The charges generated by radiation tend to collect in those regions. Consequently, lines of electric force are distorted to vary an effective sensitive area, resulting in a sensitivity variation phenomenon. In this case, also after cessation of the incident radiation, the charges having collected in the regions of space between the split electrodes are gradually swept out to cause an undesirable phenomenon of after-output.
Furthermore, when an incidence of radiation takes place at a higher rate than a charge sweep-out, the charges collect also in regions having the split electrodes formed therein. This distorts an electric potential profile in the semiconductor to raise the electric potentials adjacent the split electrodes. In one example, an amorphous selenium (a-Se) film and a common electrode are formed on a substrate having a plurality of split electrodes and thin film transistors (TFT), and signals are successively read by switching operation of the TFTs. In this particular case, a high bias must be applied to the a-Se in use, and the potential rise adjacent the split electrodes becomes high enough to influence the switching operation of the TFTs. This results in phenomena such as of slow reading operation, and causes sensibility variations or after-outputs.
The above two phenomena will particularly be described hereinafter with reference to FIGS. 1 and 2. FIGS. 1 and 2 are sectional views schematically illustrating interior conditions of a conventional radiation detector.
FIG. 1A is a schematic view showing a state before an incidence of radiation. In this state, all lines of electric force run parallel inside a thick semiconductor film 51. Where each split electrode 53 has a width “a” and a length “y” in the direction of depth, a sensitive area is “axy”. FIG. 1B is a schematic view showing a state occurring with an incidence of radiation. Of the charges (electrons and holes) generated in regions of space between the split electrodes 53, the charges (e.g. holes in FIG. 1B) moving toward the side having the split electrodes 53 are captured to stagnate (as at 55) adjacent surfaces of the thick semiconductor film 51, in the absence of electrodes for sweeping out the holes. In this way, the holes collect gradually to distort the lines of electric force in the thick semiconductor film 51. Where a space between each adjacent pair of split electrodes 53 has a width “b” and “z” in the direction of depth, a sensitive area in this case will be (a+b)×(y+z). Therefore, sensibility is varied (i.e. increased) from a×y to (a+b)×(y+z) until the region of space between the split electrodes 53 is filled with the charges.
FIG. 2 shows a construction having an amorphous selenium (a-Se) film 65 and a common electrode 67 formed on a substrate 63 having a plurality of split electrodes 61 and thin film transistor (TFT) switches. The TFT switches are operable for successively reading signals. The amorphous selenium film has a thickness d. In FIG. 2A, which is a schematic view showing a state before an incidence of radiation, an electric potential adjacent the split electrodes 61 is sufficiently low. However, when an incidence of radiation takes place at a higher rate than the charge sweep-out, the charges collect adjacent the split electrodes 61. This distorts the electric potential profile to raise the electric potential adjacent the split electrodes 61 as shown in FIG. 2B. Since a high bias must be applied to the amorphous selenium film in use, the potential rise becomes high enough to cause malfunctioning of the TFT switches. This results in phenomena such as of slow reading operation, and causes sensibility variations or after-outputs.
Such sensibility variations make a quantitative radiation detection impossible. Moreover, when the detector is used for detecting a dynamic image, a phenomenon of gradual brightness variations occurs, and incident radiation doses cause different sensitivity variation curves, resulting in a phenomenon in which an image of a preceding frame remains as an after-image.